There are numerous applications in which it is desired to form layers over substrates. For instance, it is frequently desired to form layers over semiconductor constructions during fabrication of integrated circuitry. Among the methods commonly utilized for layer formation are chemical vapor deposition (CVD) processes and atomic layer deposition (ALD) processes. A problem which can occur with CVD processes is that there is frequently less than 100% step coverage. For instance, if CVD is utilized to form a plug of material within a trench, there will frequently be a void present within the plug. The void forms because the deposition rate at the neck (top) of the trench is higher than that on the sidewall, and accordingly the entrance to the trench becomes pinched off by the depositing film before the trench is completely filled with the film.
ALD generally has better step coverage than CVD, and in some cases can come very close to, or even achieve, a voidless fill process. However, ALD can be slow, and (barring certain special cases, such as the catalytic ALD of SiO2 using tetramethylaluminum and tris(tert-butoxy)silanol) the growth rates are typically on the order of about 1 Å per cycle.
Recently, it has been proposed to utilize supercritical fluids to deliver precursors to a surface during formation of layers. A frequently utilized supercritical fluid consists essentially of, or consists of CO2.
The supercritical fluids are known to be exceptional solvents. The supercritical fluids are typically utilized by first providing a precursor within the supercritical fluid at high concentration, thus taking advantage of the solvent characteristics of the supercritical fluid. The supercritical fluid having the precursor dissolved therein is provided within a reaction chamber proximate to a substrate. Subsequently, the temperature and/or pressure conditions within the chamber are reduced so that the fluid is changed to a non-supercritical state. The fluid then lacks the solvent properties which can keep the precursor in solution, and accordingly the precursor falls out of solution and forms a layer (or a film) over the substrate.
Although supercritical fluid techniques can have advantages relative to ALD and CVD practices, there remains a need for improved methods of forming layers over substrates, and particularly for improved methods for forming layers over semiconductor substrates.